Corona-resistant solid dielectric cable

ABSTRACT

A conductor is surrounded by an inner shielding layer, an insulating layer of solid dielectric material such as polyethylene, and an outer shielding layer of polymeric material which adheres but is unbonded to the insulating layer and which has a resistivity of at least 1.0 million ohms-cm., whereby the outer shielding layer substantially suppresses the effects of corona discharge and can be readily stripped from the insulating layer for splicing or terminating the cable.

United States Patent McKean [54] CORONA-RESISTANT SOLID DIELECTRIC CABLE[72] inventor: Alexander L. McKean, Ardsley, N.Y.

[73] Assignee: Phelps Dodge Copper Products Corporation, New York, N.Y.

[22] Filed: Apr. 3, 1970 [21] Appl.No.: 33,212

[52] US. Cl ..l74/36, 174/102 SC, 174/105 SC, 174/110 B, 174/110 PM [51]Int. Cl .110) 9/02, l-lOlb 11/06 [58] Field oTSearch ..174/102,102.2,105, 105.1, 174/108, 127, 106, 106.2, 36, 110.44, 110.42

[56] References Cited UNITED STATES PATENTS 2,142,625 1/ 1939 Zoethout..174/102 X 51 Feb. 15,1972

2,453,313 11/1948 Gordon ..l74/102.2 UX 3,259,688 7/1966 Towne et al. 174/127 X 3,396,231 8/1968 Anderson ..l74/l 27 X 3,433,891 3/1969 Zysk eta1 ..174/l02 X 3,441,660 4/1969 Garner 174/102 Primary Examiner bewisl-l. Myers Assistant ExaminerA. T. Grimley Attorney-Davis, Hoxie,Faithfull & Hapgood ABSTRACT A conductor is surrounded by an innershielding layer, an insulating layer of solid dielectric material suchas polyethylene, and an outer shielding layer of polymeric materialwhich adheres but is unbonded to the insulating layer and which has aresistivity of at least 1.0 million ohms-cm, whereby the outer shieldinglayer substantially suppresses the effects of corona discharge and canbe readily stripped from the insulating layer for splicing orterminating the cable.

8 Claims, 1 Drawing Figure 4- SHlELDlNG .LAYER f-METAL SHEATH ETHYLENEPROPYLENE ISOBULYLENE ISOPRENE CROSS LINKED POLYETHYLENE.

PAIENTEBFEB 15 m2 3. 643 O04 4-SHIELDING LAYER f-METAL SHEATH ETHYLENEPROPYLENE ISOBULYLENE ISOPRENE CROSS LINKED POLYETHYLENE INVENTOR. ,4!'MNDER L. MC IKE/4N 1 CORONA-RESISTANT SOLID DIELECTRIC CABLE Thisinvention relates to electrical cables for high voltage use and moreparticularly to high-voltage cables of the type in which the insulatinglayer surrounding the conductor comprises a solid dielectric materialsuch as polyethylene (which may be cross-linked), ethylene propylene,isobutylene isoprene, etc.

High-voltage cables of the above-noted type have been largely successfuldue to the effectiveness of extruded insulation shields at the inner andouter annular surfaces of the insulating layer. These shielding layersare commonly referred to as semiconducting layers;.and a primaryrequisite of the shielding material according to the prior art is thatit be of low resistivity, preferably in the order of 100 ohms-cm. but inany case less than about 50,000 ohms-cm. With such insulation shieldingemploying so-called semiconducting material, the most efficient cabledesign provides a bond between the shielding layer and the adjacentsurface of the solid dielectric layer.

It has been found, however, that under service conditions creatingsevere corona action at theinsulation surface, these prior cables areadversely affected by corona discharge, particularly in that they sufferdeterioration or acquire instability as exhibited, for example, bychanges in corona measurements or power factor measurements. Moreover,the bond between the shielding and insulating layers makes it difficultto strip the shielding layer from the adjacent layer when splicing orterminating the cable.

An object of the present invention is to provide a cable of the typedescribed which overcomes the above-noted disadvantages.

l have discovered that by providing the outer shielding layer with aresistivity far in excess of that previously considered desirable oreven acceptable, this layer can adequately perform itsinsulation-shielding function while eliminating or at leastsubstantially suppressing the adverse effects of corona discharge, andthat the latter advantage will be obtained with the outer shieldinglayer adhering but unbonded to the insulating layer so that suchstripping of this shielding layer can be readily effected.

in a cable made according to the invention, the outer shielding layercomprises a polymeric material having a resistivity of at least 1.0million ohms-cm. Such resistivity can be considerably higher than thisminimum value. in fact, the desired results can be obtained when theresistivity is as high as 100 million ohms-cm. Also, this shieldinglayer of the new cable, although adhering this shielding layer of thenew cable, although adhering to the insulating layer, is free of bondingto the insulating layer. When the shielding layer is formed by anextrudable material, it can be provided with a smalldegree of adhesionto the insulating layer, while avoiding bonding, by properly controllingthe extruding conditions, as is well known in the art,-such conditionsincluding the extruding tempera ture and pressure, running speed, curingtube temperature and pressure, rate of cooling, etc.

The composition used for the insulation shield may be compounded fromcompositions conventionally used for so-called semiconductive shields,such as homopolymer (polyethylene) or a copolymer of ethylene. However,the proportion of carbon-black with which this composition is physicallyblended, according to the invention, is considerably less than theproportion previously used to provide the desired degree ofconductivity. For example, a standard commercial semi-conductive productsuch as Bakelites 0580 compound is an ethylene copolymer designed toprovide an electrical resistance of about 100 ohm-cm. or less,-andwhich'comprises 30-35 parts of carbon-black in a total of I parts ofblended copolymer (for instance, 35 parts of carbon-black and 65 partsof the copolymer). According to the present invention, however, thiscompound is modified to provide a resistivity of 1 million ohm-cms. orhigher, by substantially reducing the proportion of carbon-black to avalue, for example, of l0-20 parts in the total of 100 parts of theblended copolymer.

A further advantage in providing the outer shielding layer with thishigh resistivity is that its relatively low content of carbon-blackeliminates any concern about brittleness. in conventional shieldinglayers, the amount of carbon-black approaches or is at the practicalupper limit in regard to control of flexibility and brittleness.

The shielding function with this high-resistance shielding layer may beconsidered as involving. the movement of electrons under the influenceof an energy field to fill or neutralize adjacent ionized atoms. inanother sense, the high-resistance shield serves to dissipate the corona(ionization) energy. Another important advantage of this high-resistanceshield is that it remains stable (resistance remains essentiallyunchanged) under heat aging and physical flexure, whereas the lack ofthis type of stability has been a major problem with prior cables of thetype described and has limited their use.

The functioning of the present invention can be further explained by thefollowing, after noting that conventional shielding layers are actualconductive rather than semiconductive, whereas the outer shielding layerof the invention is truly semiconductive. In comparing conductiveshielding and semiconductive shielding, the true semiconductor canfunction more effectively to absorb and neutralize corona dischargeenergy, because of the relative disposition of electron orbits andenergy levels about the atomic nucleus. The electrons of any atom existin different energy levels, or orbits, about the nucleus. Each orbit canaccommodate only a certain number of electrons, so that additionalelectrons must move to a new outer orbit, and are known as valenceelectrons. As the orbits of valence electrons interreact to form bonds,their energy levels may split into energy bands called valence bands orconduction bands. In conductive materials, the valence band overlaps theconduction band, so that with the addition of only minute externalenergy (such as discharge) valence electrons are transferred to theconduction band. With insulators, a very considerable energy gap existsbetween bands, so that relatively little charge transfer occurs. Insemiconductors, the energy gap is moderate enough so that electrons canjump the gap under an external energy force. In terms of coronadischarge effects, with true semiconductors corona may be completelyabsorbed or dissipated in providing the chargetransfer energy requiredfor electrons to jump the gap from valence to conduction band. Withconductive shielding, however, charge-transfer of electrons may beeasily initiated with only minute additional increments of energy, sothat total corona energy is not readily absorbed. Hence the coronadischarge is rendered relatively innocuous and harmless with truesemiconductors, but not with conductive shielding.

The invention will be described further in connection with theaccompanying drawing, in which the single illustration is across-sectional view of one embodiment of the new cable.

As illustrated, the high-voltage cable comprises a central conductor 1,which may be solid or stranded copper or aluminum. Immediatelysurrounding this conductor and extruded thereon is an inner shieldinglayer 2. Extruded over the shielding layer 2 is surrounding insulatinglayer 3 of solid dielectric material, such as ethylene propylene orisobutylene isoprene but preferably polyethylene (which may becross-linked). lmmediately surrounding the insulating layer 3 andextruded thereon is an outer shielding layer 4 having a resistivity ofat least one million ohm-cm., preferably somewhat greater but not inexcess of I00 million ohm-cm. The shielding layer 4 is closelysurrounded by the usual metallic sheath 5.

The shielding layer 4 may be compounded in accordance with the examplepreviously described. As previously mentioned, this layer is not bondedto the insulating layer 3 but adheres thereto, so that it can be easilyremoved incident to splicing or terminating the cable; and it will beunderstood from the foregoing that the unbonded adhesion can be effectedin a conventional manner by controlling the conditions under which theshielding layer 4 is extruded on the insulating layer 3.

The inner shielding layer 2 is preferably well bonded to the surroundinginsulation 3, since this has the effect of avoiding corona. Such a bondcan be obtained by using a conventional technique incident to extrudingthe insulation 3 over the layer 2. The composition used for theshielding layer 2 may be conventional; that is, it may have aresistivity in the order of 100 ohms-cm, for example. However, I preferto use for this layer a composition of high resistivity similar to thatused for the outer shielding layer 4. In this way, if the bonding of thelayers 2 and 3 is not perfect and voids form at their interface, theconductor shield 2 will perform better than in the case of aconventional shielding layer.

As an example of the effectiveness of the present invention, a length of69 KV power cable having a conventional insulating layer 3 ofcross-linked polyethylene, but with high-resistance shielding layers 2and 4 as previously described, was '5 load cycled under twice the ratedvoltage to conductor temperatures up to 85 C and 105 C. for severalmonths. Although limited corona discharge occurred during the testperiod, the cable did not show any evidence of significant instabilityeither on the basis of change in corona measurements or power factormeasurements. Physical (dissection) examination after testing likewisefailed to reveal any evidence of deterioration.

Although I have referred to the shielding layers 2 and 4 as being formedby extrusion, it will be understood that they may be formed by a tape ortapes of the high-resistance polymeric material and which can be appliedin lapped or butted fashion.

lclaim:

1. In a high-voltage electrical cable, the combination of a conductor,an inner shielding layer surrounding the conductor, an insulating layerof solid dielectric material surrounding said shielding layer and havingan outer surface. and, adhering to said outer surface but free ofbonding thereto whereby it is readily stripable therefrom, an outershielding layer of polymeric material surrounding the conductor andhaving a resistivity of at least 1.0 million ohms-cm, said insulatingand outer shielding layers extending along substantially the entirelength of the conductor whereby said outer shielding layer is operableto substantially suppress the effects of corona discharge.

2. The combination according to claim 1, in which said inner shieldinglayer is bonded to the insulating layer.

3. The combination according to claim 2, in which said inner shieldinglayer has a resistivity of at least 1.0 million ohms-cm.

4. The combination according to claim 1, in which said outer shieldinglayer is the homogeneous product of extruding said shielding materialaround the insulated conductor.

5. The combination according to claim 1, in which said outer shieldinglayer comprises said shielding material in tape form.

6. The combination according to claim I, in which said insulating layeris a polymeric material.

7. The combination according to claim 1, in which said insulating layeris polyethylene.

8. The combination according to claim 1, in which said insulatingmaterial is an elastomer.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent Noo3,643,004 Dated February 15, 1972 Inventor(s) Alexander L. McKean It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Cover page, item (22) "Filed: Apr. 5, 1970" should be -Filed: Apr, 22,l970-.

Column 1, lines 48 and 49, after "adhering",

delete "this shielding layer of the new cable although adhering".

Signed and sealed this 19th day of September 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Commissioner of PatentsAttesting Officer 3 v UNITED STATES PATENT OFFICE QERTWICATE UFCOREQ'HQN Patent No 3,643,004 D ed Februarv 15, 1972 Inventor(s)Alexander L. McKean It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Cover page, item (22) "Filed: Apr. 3 1970" should be Filed: Apr 3,l970-.,

Column 1, lines 48 and 49, after "adhering",

delete "this shielding layer of the new cable although adhering". I

Signed and sealed this 19th day of September 1972,

(SEAL) Attest:

EDWARD M.FLE'I'CHER,JR. ROBERT GOTTSGHALK Attesting Officer Commissionerof Patents

2. The combination according to claim 1, in which said inner shieldinglayer is bonded to the insulating layer.
 3. The combination according toclaim 2, in which said inner shielding layer has a resistivity of atleast 1.0 million ohms-cm.
 4. The combination according to claim 1, inwhich said outer shielding layer is the homogeneous product of extrudingsaid shielding material around the insulated conductor.
 5. Thecombination according to claim 1, in which said outer shielding layercomprises said shielding material in tape form.
 6. The combinationaccording to claim 1, in which said insulating layer is a polymericmaterial.
 7. The combination according to claim 1, in which saidinsulating layer is polyethylene.
 8. The combination according to claim1, in which said insulating material is an elastomer.